1. Field of the Invention
The present invention relates generally to bearings, and more specifically to a bearing load maintainer, primarily for use in spacecraft applications for maintaining a hard preload during launching operations and for thereafter upon bearing wear, maintaining a preselected minimum spring-biased soft preload.
2. Prior Art
Ball bearings in spacecraft applications are normally used in pairs and are preloaded axially toward each other to remove the play or looseness in them. Besides providing some rigidity to the shaft on which the bearings are mounted, the preload on the bearings minimizes the sliding of the balls in the raceway ball grooves. The sliding of the balls is detrimental to the lubrication system and the life of the bearings. However, an excessively large preload also tends to reduce the useful life of solid film lubricated bearings, as well as liquid or grease lubricated ones if their operation is in the boundary regime where ball-to-race contact is made.
Many designs of spacecraft mechanisms specify high axial preloads to prevent one or more of the bearings from becoming unloaded during launch vibration. It is believed by some designers that such unloading might cause impacts within the bearings that would result in the formation of ball brinell impressions that would then result in jittery operation and possibly reduced bearing life. Once the hardware is in the zero G environment of earth orbit, the high preloads are no longer useful and in fact are detrimental. Therefore, the axial preload on the bearings should be kept to the minimum that is necessary to remove the axial play. Generally speaking, there are two types of axial preloads. There is a hard preload applied by means of nuts, bolts or screws which force the ground bearing races against each other or against precision ground separation spacers. There is also a soft preload applied by compliant members such as springs which apply an axial load depending upon the spring rate and the displacement of the compliant members.
All prior art known to the applicant falls into one of the two aforementioned categories, namely hard preloaded bearings or soft preloaded bearings. Consequently, there is an inherent disadvantage in the use of prior art bearing configurations for space applications. More specifically, hard preloaded bearings are suitable for the rocket launch portion of a spacecraft maneuver because the hard preload withstands the high vibration load normally incurred during launch. Unfortunately however, during earth orbiting over many years, hard preloads as previously noted tend to reduce the useful life of ball bearings. More specifically, when the preload effect is lost due to wear and or differential thermal expansion, this results in loss of shaft alignment precision which can result in premature mechanical failures. On the other hand, while soft preload bearing configurations are preferred to maintain accurate alignment of rotating assemblies during years of a spacecraft orbit life, spring loaded bearings sets are readily damaged by chatter during launch if the preload magnitude is low or they exhibit high wear and fatigue if the preload magnitude is high.
There is therefore, an existing need for a bearing load maintainer which provides the stiffness of a duplex set, that is, a hard preload set when new, thereby avoiding damage from chatter such as during rocket launch and which also provides the compliance of a spring loaded set as the bearing wears or unloads due to differential thermal expansion thereby preserving the alignment precision of the bearing set. The applicant knows of no prior art which solves the aforementioned long felt need by providing such dual load bearing capabilities. More specifically, the following prior art is deemed to be the most relevant presently known to the applicant.
U.S. Pat. No. 4,699,528 is directed to a rather standard prior art rotary assembly having a self-positioning bearing. Axial supports consist of the snap-ring at one end of the ball bearing and the combination of a snap-ring and spring at the other end. The overall effect is to resiliently urge the inner ring and outer ring axially toward each other. Axial play of the balls between the two raceways is thus obviated regardless of the temperature/wear factors.
U.S. Pat. No. 4,173,376 is directed to bearing assemblies where bearing pre-loads are exerted by both high/low rate non-linear springs in tandem. At opposing ends of a shaft are main spring washers and soft spring washers. Main spring washers are formed of a high rate spring constant and soft spring washers are of a low rate spring constant. The spring washers are loaded axially inward toward each other to some predetermined pre-load by means of a nut. The resulting load is applied to the inner races of bearings through the thrust members.
U.S. Pat. No. 2,068,594 is directed to bearing structures for rotatable spindles. Pre-loading of the bearing is provided by insertion of a stiff spring over the threaded end of the spindle into abutting relation with the inner race of the upper bearing. The degree of pre-loading is determined by the manipulation of a nut which may be held against rotation relative to the spindle following adjustment.
U.S. Pat. No. 3,578,829 is directed to what is termed a bottom bracket for a bicycle. A ring-like spacer is in pressure contact with one end of the coil spring. The spacer is positioned inwardly of the second ball bearing race which is a left side bearing slideably mounted on the peripheral surface of the crank shaft. The ball bearing race is in sliding contact with the shaft and has a ring-like configuration with an inwardly depressed or concave annular steel ball bearing surface. The adjusting cap is threadedly fitted into the screw head on one end of the bracket lug.